Metabolism of cholesterol, as measured by disappearance of H3 cholesterol from the blood, its excretion and exchange with the tissue cholesterol decreases during aging. Mechanisms of this decrease will be studied because of the obvious importance of cholesterol metabolism in the atherosclerosis which is mainly a disease of older people. Parabiotic connection of young and old animals speeds up metabolism of cholesterol in the old partner. This cannot be achieved by therapy with cholesterol-handling hormones because cholesterol metabolism in old animals is very resistant to the action of these hormones. Mechanism of the effect of parabiosis on cholesterol metabolism is under investigation.